Most magnetic recording media which are commercially available at present are of a longitudinal recording type, in which most of easy-magnetization axes in a magnetic film are oriented horizontally with respect to a substrate.
In order to increase recording density in such a longitudinal magnetic recording medium, the size of magnetic particles must be decreased to reduce noise. However, when the size of a magnetic particle is decreased, the volume of the particle is also decreased, and consequently, reproduction characteristics of the medium may deteriorate due to thermal instability. In addition, when recording density is increased, the effect of a diamagnetic field at a recording bit boundary may cause an increase in medium noise.
In contrast, in a perpendicular magnetic recording medium, in which most of easy-magnetization axes in a magnetic film are oriented vertically with respect to a substrate, the effect of a diamagnetic field at a recording bit boundary is minimal even when recording density is increased, and recording magnetic domains having clear boundaries are formed, thereby enabling noise reduction.
In addition, even when the volume of a magnetic particle is relatively large, recording density can be increased, and thus thermal stability can be enhanced. Therefore, a perpendicular magnetic recording medium has recently become of interest.
For example, Japanese Patent Application Laid-Open (kokai) No. 60-214417 discloses a perpendicular magnetic recording medium including a perpendicular magnetic film formed from a Co alloy, and an undercoat film formed from Ge or Si.
Japanese Patent Application Laid-Open (kokai) No. 63-211117 discloses a perpendicular magnetic recording medium including a perpendicular magnetic film formed from a Co alloy, and a film (thickness: 1 to 100 Å) formed from a carbon-containing material, serving as an undercoat film of the magnetic film.
However, the aforementioned conventional magnetic recording media encounter difficulty in enhancing squareness ratio, and involve a problem that a nucleation field (Hn) is lowered. Therefore, the media exhibit poor thermal stability when recording density is decreased.
In contrast, magnetic recording media which enable enhancement of Hn have been proposed, each of such media including a multi-layer film containing a plurality of layers of transition metal (e.g., Co) and layers of noble metal (e.g., Pt) (Japanese Patent Application Laid-Open (kokai) Nos. 6-111403 and 8-30951, and U.S. Pat. No. 5,660,930).
In recent years, there has been demand for magnetic recording media of higher recording density, and in accordance with this trend, further improvements to noise characteristics have been required.
However, the aforementioned conventional magnetic recording media (each including a multi-layer film containing transition metal layers and noble metal layers) are not satisfactory in terms of noise characteristics, and thus there has been demand for a magnetic recording medium that exhibits excellent noise characteristics.